In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically, comprise a top layer applied over the surface of a hydrophilic substrate. The top layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the exposed regions are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate is called a positive-working printing plate. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate is called a negative-working plate. In each instance, the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and the regions of the radiation-sensitive layer (i.e., the image areas) that remain repel water and accept ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Negative-working, alkaline developable imageable elements that comprise compounds that form an acid on thermal imaging have been developed for use with infrared lasers. For example, Haley, U.S. Pat. No. 5,372,907, discloses a radiation-sensitive composition that is sensitive to both ultraviolet and infrared radiation. The composition comprises (1) a resole resin, (2) a novolac resin, (3) an acid generator, and (4) an infrared absorber. Typically, the acid generators are onium compounds, such as 2-methoxy-4-aminophenyl diazonium hexafluorophosphate, phenoxyphenyldiazonium hexafluoroantimonate, anilinophenyldiazonium hexafluoroantimonate, bis-4-dodecylphenyliodonium hexafluoro antimonate, diphenyliodonium hexafluorophosphate, diphenyl iodonium hexafluoroantimonate, diphenyl iodonium hexafluoroarsenate, dicumyliodonium hexafluorophosphate, and triphenylsulfonium hexafluoroantimonate.
Although acid generation in these systems is relatively efficient with ultraviolet imaging, it is inefficient with infrared (830 nm to 1200 nm) imaging.
Because the compositions comprise four components, formulation of the composition is complicated. The acid generators are typically onium salts in which the anion contains an element such as antimony or arsenic, producing handling and disposal problems. Thus, a need exists for a more efficient method for forming images by direct digital imaging with infrared radiation and in which the acid generator does not contain elements, such as antimony or arsenic, that present handling and disposal problems.